/Engine/Source/Editor/UnrealEd/Public/ThumbnailHelpers.h
class FMaterialThumbnailScene : public [FThumbnailPreviewScene](API\Editor\UnrealEd\FThumbnailPreviewScene)
bool
bForcePlaneThumbnail
Material being used for something that only makes sense to visualize as a plane (UI, particle sprites)
AStaticMeshActo...
PreviewActor
The static mesh actor used to display all material thumbnails